Laughing gas no laughing matter for climate change

There’s an interesting article published in PLOS One that I like. It’s one of these things that’s very clever, but the basic idea is very simple.

Temperatures are rising, and there’s plenty of research on how that might affect plants. In PLOS One this month Gschwendtner et al. investigate how rising temperatures affect the soil. In fact they look at the microbe community in it. Bacteria and archaea are part of the biological process of putting Nitrogen into usable form for plants. Knowing how they might react to climate change would be useful.

The experiment was very simple. At the Tuttlingen Research Station in southern Germany, Gschwendtner and her team took some beech seedlings, and the soil around them, growing on a northwest facing slope and replanted some of them on a southwest facing slope. They got more sunlight on the soil and so you effectively change the climate for those soil samples. Compare one with the other and you get to see what sort of changes warmer weather might have.

That sounds simple, but there are some obvious problems. If the geology of the new slope is radically different, maybe you’re just measuring the change in geology, not climate. So what they did was core the soil, to make sure the new sites were a close match for the old sites.

That’s fine, but there’s another problem. The moved seedlings will have moved. That might lead to stresses that the other seedlings didn’t have. If that’s the case you’re measuring stress not climate. So to get round that problem they also replanted the control sample in new locations on the northwest slope, so they had the same stresses too.

The target was to see how Nitrogen production in the soil was affected. Measuring the soil and sniffing for outgassing would be a pain, so they used a different technique. They tested the soil for specific genes. Sampling the soil and comparing the relative proportions and quantities of certain genes in the soil would give measure of the kind of activity going on. For example they looked for the genes nirK, nirS, cnor and nosZ as markers of denitrification. These are genes associated with microbes that take nitrates in the soil and convert them to gases. If there are more bacteria and archaea working on denitrification, then they will be more copies of these genes to find.

What they found is that these genes became much more common in soil samples from the seedlings moved to the sunnier position. They also followed up the experiment by simulating drought and flood. They found that the denitrifying microbes did better under those conditions.

This has a double blow for plants. The first is that the plants are competing with these microbes for nitrogen. We think of plants living off carbon dioxide and water, but building proteins needs nitrogen too. The second blow is that the nitrogen is lost from the soil when the microbes emit it as nitrous oxide N₂O. It’s known as laughing gas, but it’s also a greenhouse gas, adding to the climate problems the plants are already facing.

I think what appeals to me about the paper is the clever way they’ve looked at denitrification. If I wanted to measure change of nitrogen in a soil, I’d try directly measuring the nitrogen. Looking for DNA markers is simpler, and it also gives an idea of what might be driving that change. I also like the simplicity of the idea let’s move seedlings from here to there, and the fact that the control was replanted too. With hindsight it’s easy to say that should be done, but I bet that wouldn’t have occurred to me until the experiment was near its end.

As it’s in PLOS One you can pick it up now as an Open Access paper.

Gschwendtner S., Tejedor J., Bimueller C., Dannenmann M., Kögel Knabner I. & Schloter M. (2014). Climate Change Induces Shifts in Abundance and Activity Pattern of Bacteria and Archaea Catalyzing Major Transformation Steps in Nitrogen Turnover in a Soil from a Mid-European Beech Forest, PLoS ONE, 9 (12) e114278. DOI: http://dx.doi.org/10.1371/journal.pone.0114278